One method of thermal power generation is by gas turbines that are powered by the combustion of some fuel, and it may be applied to power plants having a capacity up to several hundreds of thousand of kilowatts. Additionally, there are methods of thermal power generation that are commonly called combined cycle power generation. These methods are based on a gas turbine cycle and a steam cycle in combination. The steam cycle is powered by the exhaust energy from the gas turbine cycle. They offer an advantage of being started and stopped easily and allow easy control for load variation. Moreover, these combined methods of power generation are more efficient than the gas turbine cycle or ordinary steam power generation alone
The general trend in combined cycle power generation in regions where natural gas availability is an issue is toward firing the gas turbine with revaporized liquefied natural gas which is a clean fuel. Combined cycle power generation involving liquefied natural gas will become more important in the countries and areas where energy demand is expected to increase in the future. Liquefied natural gas is desirable from the standpoint of stable supply in view of its huge reserves and also from the standpoint of environmental protection.
In the meantime, a gas turbine has the disadvantage of decreasing in output with increasing atmospheric temperature. This is caused by an increase in atmospheric temperature that decreases the density of air being supplied to the gas turbine. The lower the density of air, the smaller the mass of air in the same volume. Unfortunately, atmospheric temperature is highest in the afternoon in summer when the electric power consumption and demand can be high due to increase in operation of equipment such as air conditioners. In other words, the output of gas turbines is lowest when the electric power consumption is highest. This prevents high efficiency of the gas turbine cycle for power generation. As such, there have been attempts to improve the efficiencies of gas turbine plants. In one method, it has been proposed to extend a gas turbine plant with a waste-heat boiler and to combine the gas turbine plant with a steam turbine plant. The gas turbine and the steam turbine each drive their own generator or drive a single generator via a common shaft. These combination plants, referred to as combined cycle plants, are generally distinguished by their very good energy conversion efficiencies which range in the order of magnitude from 50 to 58%. These high efficiencies result from the cooperation of a gas turbine with at least one steam turbine plant. The gas turbine exhaust gases are passed through a waste-heat boiler and the residual heat potential of these waste-gases is utilized for producing the steam required for feeding the steam turbine. LNG has been used in combined cycle plants as a combustion energy source.
LNG is normally transported as a cryogenic liquid in specialized vessels. At the receiving terminal this cryogenic liquid, which is approximately at atmospheric pressure and at a temperature of around −260° F., has to be regasified and fed to a distribution system at ambient temperature and at a suitably elevated pressure, typically ranging up to 80 atmospheres. The liquid may be pumped to the required pressure so that when heat is added and it is regasified, no compression of the resultant natural gas is required.
Although many suggestions have been made and some installations have been built to utilize the large cold potential of the LNG, in most receiving terminals the cold potential is wasted and the LNG is simply heated with a large flow of sea water which has to be applied in such a manner as to avoid ice formation.
At a few terminals, the cold potential is utilized in air separation plants or similar cryogenic installations or for refrigeration purposes in the freezing and storing of foodstuffs. It has also been proposed to use the cold LNG as a heat sink in a power cycle to generate electrical energy. A number of possible cycles have been proposed which seek to overcome the difficulties caused by the large temperature difference through which the LNG is heated and the particular shape of the warming curve. However, it has been found that even with relatively simple cycles only a small part of the available cold potential can be utilized. Proposals to increase the efficiency employ more complex cycles involving a large number of turbines operating between different pressure levels.
Accordingly what is needed is a gas turbine system that offers increased efficiencies as to prior art gas turbine systems. Also what is needed is a gas turbine system that utilizes liquefied natural gas as a heat sink as well as a possible source of fuel for the turbine system.